The video portion of color television signals includes both luminance and chrominance information. The luminance information relates to the monochome portion of the signal, whereas the chrominance information refers to the hue and saturation of the signal. The chrominance information involves two variables (usually designated I and Q), which are used to amplitude and phase modulate a subcarrier signal which, in the United States, has a frequency of 3.58 MHz.
A television receiver employs synchronous detectors to demodulate the subcarrier signal in order to detect the I and Q variables. The chrominance portion of the transmitted signal also includes a phase reference to be used by the subcarrier regenerator or local oscillator of the synchronous detectors of the receiver to reproduce the 3.58 MHz subcarrier. The phase reference, commonly called the color-synchronizing burst, or simply color burst signal, is a short sample of unmodulated subcarrier transmitted during the horizontal blanking period after each horizontal sync pulse. The subcarrier regenerator or local oscillator, which is usually a stable, quartz crystal-controlled oscillator, is phase-locked to the color burst signal once each television line.
If the video signal is recorded on a video tape recorder, the color burst signal is frequently degraded in phase or frequency as a result of recorder wow or flutter. If the degradation is significant, the crystal-controlled local oscillator may not be capable of maintaining a phase lock with the burst signal. If a lower Q oscillator is used, such as an LC oscillator, the oscillator may drift between burst signals thereby resulting in color distortion. In addition, the LC oscillator output frequency may drift sufficiently from the subcarrier frequency such that the oscillator locks on a color burst sideband signal which differs from the color burst signal by multiples of the line frequency. In either case, the television receiver picture color is unacceptable.
In order to compensate for degradation in the video signals produced by video tape recorders, such recorders sometimes utilize chroma stabilizers or color correction circuits. One such circuit is commonly referred to as a time base corrector which compensates for wow and flutter by adding or subtracting a delay in the recorder output. Another approach is called heterodyning, which utilizes a burst continuation oscillator. Both of these approaches are unsatisfactory in that they significantly increase the cost of the video tape recorder. In addition, the heterodyning circuit significantly reduces the video bandwidth, thereby causing the picture to smear or appear out of focus.
The present invention overcomes the above-noted limitations of chroma stabilizers. The disclosed invention is incorporated in the receiver rather than in the recorder and may be implemented at low cost. In addition, the video bandwidth is not reduced. These and other advantages of the present invention will become apparent to those skilled in the art after having read the following Best Mode for Carrying Out the Invention, together with the drawings.